Move physics body definition to DrawableEntity

This is more sensible, and in preparation for state machines and
elimination of child-classes of DrawableEntity
Cleaned up debug circles, made them toggleable.

Having the physics engine handle debug circles makes sense, and it can
adjust the quality of the circles so that pikmin circles don't overload
the polygon list. Would be nice to not draw the circles extra times
(currently they're drawn every pass) but not a big deal.